1. Field of the Invention
The present invention relates to butterfly-type control valves, and in particular to butterfly-type control valves having non-circular valve members.
2. Description of the Related Art
DE19848440A1 teaches a butterfly-type control valve, in which a substantially rectangular valve member and a valve shaft are made of synthetic resin and formed integrally with each other by an injection molding process. A valve housing that accommodates the valve member also is made of synthetic resin and formed by an injection molding process. The valve member has opposing longitudinal sides that extend in the direction of the axis of the valve shaft. Because the valve member and the valve body are made of synthetic resin and are formed by injection molding processes, it is necessary to take into consideration of molding tolerances and shrinkage of molded products due to cooling after the molding processes. Therefore, the above publication proposes to join a valve edge part to the valve member through an injection molding process after the valve member has been molded. In addition, a valve mounting portion of the valve housing has a special configuration in order to compensate for shrinkage of the valve shaft.
However, the control valve of the publication still involves a problem, because the valve housing that accommodates the rectangular valve member has a long length in the direction of the valve shaft in comparison with the width (height) of the valve housing. Thus, when the valve housing has been cooled after the molding process, a wall part of the valve housing having a long length may warp inwardly toward the valve member due to the shrinkage.
The same problem also may occur in case that a valve member is not made of synthetic resin but is made of metal. FIG. 3 shows a known butterfly-type control valve that has a metal valve member. The control valve of FIG. 3 has a valve housing 22 that is made of synthetic resin and is formed by an injection molding process. The valve housing 22 defines a flow channel 23. A valve member 25 is disposed within the flow channel 23. The valve member 25 is made of metal and has a substantially rectangular configuration. The valve member 25 is mounted on a valve shaft 26 that is made of metal. Both ends of the valve shaft 26 is rotatably supported by the valve housing 22.
Horizontal wall portions 22b of the valve housing 22 that define the upper and lower walls of the flow channel 23 as viewed in FIG. 3 have a length that is greater than the length of vertical wall portions of the valve housing 22 that define the right and left walls of the flow channel 23.
Therefore, when the valve housing 22 has been cooled after it has been molded by the injection molding process, the shrinkage of the horizontal wall portions 22b is greater than the shrinkage of the vertical wall portions. As a result, the horizontal wall portions 22b may warp inwardly toward the flow channel 23 as exaggeratedly indicated by chain lines in FIG. 3.
As a result, the valve member 25 may contact the wall portions 22b before it reaches a predetermined full-close position.